I've used it on my own recordings because sometimes you have a magical, inspired take with a pitch problem that's better than all your other takes that are perfectly on pitch. You can also use it to thicken a vocal by leaving the original and the pitch corrected version together in the mix. It creates a chorus effect. To do this, the original has to be basically on pitch. Still, humans don't ever reproduce frequencies perfectly, which allows you to create the effect. (There are amazing singers who can come close.)

rap songs? they dont need to harmonise so i doubt its used that often though i do see a massive difference between studio and live but thats down to most rappers feeling the need to shout on stage rather than just turning the mic up